The Supreme Court ruled unanimously Monday that police must obtain a search warrant before using a GPS device to track criminal suspects. But the justices left for another day larger questions about how technology has altered a person’s expectation of privacy.

Justice Antonin Scalia wrote that the government needed a valid warrant before attaching a GPS device to the Jeep used by D.C. drug kingpin Antoine Jones, who was convicted in part because police tracked his movements on public roads for 28 days.

“We hold that the government’s installation of a GPS device on a target’s vehicle, and its use of that device to monitor the vehicle’s movements, constitutes a ‘search’ ” under the Fourth Amendment’s protection against unreasonable searches and seizures, Scalia wrote.

All justices agreed with the outcome of the case, which affirmed a panel of the U.S. Court of Appeals for the D.C. Circuit that said evidence of Jones’ s frequent trips to a stash house where drugs and nearly $1 million in cash were found must be thrown out.

The Supreme Court’s recent ruling overturning the warrantless use of GPS tracking devices has caused a “sea change” inside the U.S. Justice Department, according to FBI General Counsel Andrew Weissmann.

Mr. Weissmann, speaking at a University of San Francisco conference called “Big Brother in the 21st Century” on Friday, said that the court ruling prompted the FBI to turn off about 3,000 GPS tracking devices that were in use.

These devices were often stuck underneath cars to track the movements of the car owners. In U.S. v. Jones, the Supreme Court ruled that using a device to track a car owner without a search warrant violated the law.

After the ruling, the FBI had a problem collecting the devices that it had turned off, Mr. Weissmann said. In some cases, he said, the FBI sought court orders to obtain permission to turn the devices on briefly – only in order to locate and retrieve them.

Mr. Weissmann said that the FBI is now working to develop new guidelines for the use of GPS devices. He said the agency is also working on guidelines to cover the broader implications of the court decision beyond GPS devices.

For instance, he said, agency is now “wrestling” with the legality of whether agents can lift up the lid of a trash can without committing trespass. The majority opinion in U.S. v. Jones held that the agents had trespassed when placing the GPS device on a car without warrant.

He said the agency is also considering the implications of the concurring justices – whose arguments were largely based on the idea that a person has a reasonable expectation of privacy in the totality of their movements, even if those movements are in public.

“From a law enforcement perspective, even though its not technically holding, we have to anticipate how it’s going to go down the road,” Mr. Weissmann said.